Thermal transfer layer

ABSTRACT

The invention concerns a thermal transfer layer with high transfer coefficient between two materials which can have different expansion coefficients. The thermal transfer layer comprises expanded graphite inserted between the materials which are selected from among carbonaceous materials, ceramics and metals or metal alloys. The expanded graphite is either inserted in the form of a rolled or compressed sheet, or is compressed in situ. The invention also concerns a device for the cooling of a structure subjected to intense, continuous, intermittent or pulsating heat flux, by means of fluid circulation tubes placed in the passages in the structure. A flexible material which is a good heat conductor in a compressed state, such as expanded graphite, is placed between the structure to be cooled and each tube.

BACKGROUND OF THE INVENTION

The present invention concerns a thermal transfer layer with hightransfer coefficient between two materials which can have differentexpansion coefficients, and its application to the cooling of astructure subjected to intense heat flux.

The materials of the invention are selected from among carbonaceousmaterial, ceramics and metals or metal alloys. Carbonaceous materials inthis sense include essentially industrial carbons graphites, andcarbon-carbon composites.

STATE OF THE ART

It is generally known that, particularly if they have differentexpansion coefficients, such materials can be joined with a good thermaltransfer only with some difficulty, and this lack of transfer can bedetrimental in numerous applications.

Thus for example, in devices wherein an element of carbonaceous materialmust be cooled by water circulation, the fragility and porosity of thecarbonaceous material generally prevent direct cooling and the elementmade of carbonaceous materials must be affixed to a metal element.Problems then arise concerning the mechanical fixation and the thermalcontact.

Numerous cases are known wherein a structure subjected to intense heatflux is to be cooled, wherein this heat flux can be of external orinternal origin in relation to the structure, which can be either ametal structure or a carbonaceous or ceramic material. The heat flux,according to circumstances, can be continuous, intermittent orpulsating.

Chemical reactors, combustion devices, continuous casting of moltenmetals, both fission and fusion nuclear reactors, targets which aresubjected to high radiation fluxes (X-rays, lasers, etc.), or particles,which can be either continuous or pulsating, could be citedparticularly. The traditional solution consists of placing assemblies oftubes in which a coolant fluid circulates inside the structure to becooled.

The problem then is to obtain very good thermal transfer, in the courseof the thermal cycles to which the structure is being subjected, betweenthe passages in the structure and the external walls of the metalcooling tubes, despite the irregularities of the contacting surfaceswhich are often very rough (for instance in fusion thermonuclearreactors) and especially the different expansion coefficients of thetubes and the structure to be cooled.

In order to have an acceptable thermal transfer, clamping pressureshigher than 100 kPa must be exerted between the components. Under thebest circumstances, transfer coefficients on the order of 9×10³ W.m⁻².K⁻¹ are obtained. Transfer coefficients are very sensitive to thesurface conditions of the elements and can be reproduced only with thegreatest difficulty, which is obviously quite awkward.

One method to resolve this problem is brazing. This solution, which isvery effective with some materials, is costly and requires a temperaturebelow the fusion temperature of the brazing process. Moreover, for thematerials having very different expansion coefficients, it is possiblein some cases to braze them by insertion of a metal sheet whichaccommodates the stresses. Then is it necessary to use costly anddelicate metals such as molybdenum, zirconium, etc.... Finally, ceramicssuch as silicon carbide and nitride are extremely difficult to braze,especially if they are calcined to a density near theoretical.

The main purpose of the invention is to provide a thermal transfer whichis simpler to utilize, more economical and which can be used at hightemperature (above 2000 degrees C if the materials to be joined allowit).

SUMMARY OF THE INVENTION

A first object of the invention is a thermal transfer layer with hightransfer coefficient between two materials which can have differentexpansion coefficients, characterized in that it is constituted ofexpanded and recompressed graphite, inserted between the materials to bejoined.

The materials to be joined thermally are selected from among:

carbonaceous materials: artificial carbons and graphites such asvitreous carbon, polycrystalline graphites, etc. ..., carbon-carboncomposites.

ceramics such as silicone carbide, silicon nitride, boron carbide,tantalum carbide,

metals and metal alloys.

According to the invention, the thermal transfer layer, for instance,can be inserted between two different carbonaceous materials, or acarbonaceous material and a metal, or a ceramic and a metal.

A second object of the invention is an arrangement for cooling astructure subjected to intense, continuous, intermittent or pulsatingheat flux, by means of tubes for the circulation of fluid, placed inpassages in the structure, characterized in that a flexible materialwhich is a good heat conductor in compressed state, and can be chargedwith a metal or a carbonaceous powder, is inserted between each tube andthe structure to be cooled. This flexible material can advantageously beconstituted of expanded graphite, which is more or less recompressed orrolled. It can also be constituted of other forms of flexiblecarbonaceous materials, such as woven materials and felts of carbon orgraphite fiber which may be charged with metal powder.

A third object of the invention is a process for cooling a structuresubjected to intense, continuous, intermittent or pulsating heat flux,by means of fluid circulation tubes, placed in passages in thestructure, characterized in that each tube is surrounded beforehand witha layer of flexible material as defined in the preceding, then each tubeis inserted into the passages, and the tubes are subjected to expansionunder pressure so as to ensure the compression of the flexible materialbetween the tube and the passages to at least 10 kPa.

A fourth object of the same invention is a process for cooling astructure subjected to intense, continuous or pulsating heat flux, bymeans of fluid circulation tubes, this structure constituted of aplurality of discrete elements, characterized in that at least onesemi-circular passage is formed in each element of the structure, eachelement is placed on at least one cooling tube, with insertion of alayer of flexible material as defined in the preceding and the elementsand the tubes are interlocked, so as to apply to the flexible material adegree of compression equal at least to 10 kPa.

Finally, a last object of the invention is its application to thecooling of the first wall of a fusion thermonuclear reactor,particularly of the "TOKAMAK" type.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 are cross-sectional views illustrating the thermaltransfer layer of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For clear comprehension of the invention, it is to be recalled thatexpanded graphite is obtained by abrupt heating of foliated graphite,even to as high as 1000 degrees C, thus giving an exfoliated graphitehaving a density on the order of 0.002. This graphite can then be moreor less recompressed in blocks which are of density from 0.02 to 2 orrolled in sheets of 0.1 to 2 mm thickness, of density on the order of 1.The expanded recompressed graphite possesses an excellent heatconductivity in the compression plane and a heat conductivity which ismuch lower in the perpendicular direction. But it also presents goodflexibility and good elasticity. Because of this, it allows for aconductive contact even for great thermal strains or deformations of thematerials.

This products, called "PAPYEX", is available from "LE CARBONE LORRAINE"company. It exists in different gradations, differing particularly intheir expansion coefficients, the expansion coefficients being betweenapproximately 4 and 6.10 ⁻⁶ K ⁻¹. This expanded graphite can alsocomprise a charge such as a metal powder, which improves its thermalconductivity.

Various tests show that the thermal transfer coefficient between twosurfaces of materials which can have different expansion coefficients isalways improved when expanded compressed graphite is inserted betweensaid surfaces. This gain depends upon the temperature, the clampingpressure of the materials, their surface condition and theirconstitution.

The expanded graphite according to the invention can be inserted incompressed state in the form of compressed or rolled sheet.

Graphite can also be inserted in a non-compressed form and subsequentlycompressed in situ during application of the materials. This lastvariation is used advantageously when the surfaces of the materials arenot flat and/or are very rough. In the following non-limiting examples,the term "expanded graphite" generally designates exfoliated graphite,more or less recompressed or rolled.

EXAMPLE 1

A cylindrical disc of graphite of 50 mm diameter is applied to a metalsurface by central support.

The graphite (grade 1346 of "LE CARBONNE-LORRAINE") has an expansioncoefficient of 5.5 to 6×10 ⁻⁶ .K ⁻¹.

The metal is of stainless steel 316L which has an expansion coefficientof 16×10 ⁻⁶ .K ⁻¹.

Comparative tests are carried out to determine the heat transfercoefficient between the graphite and the metal surface using a powerapplication of 75 watts. In some examples, the graphite directlycontacts the stainless steel, while in other examples, a thermaltransfer layer according to the invention is located therebetween. Thisthermal transfer layer is a sheet of expanded graphite having a density1 and thickness 0.2 mm which is placed under various applicationpressures, as noted.

The results are shown in Table 1 hereinafter.

                  TABLE 1                                                         ______________________________________                                        kPa                                                                           pressure   10           80       120                                          ______________________________________                                        A in                                                                          W.m.sup.-2 K.sup.-1                                                                      1,7 × 10.sup.3                                                                       1,4 × 10.sup.3                                                                   2,2 × 10.sup.3                         B in                                                                          W.m.sup.-2 K.sup.-1                                                                      2,7 × 10.sup.4                                                                       3,4 × 10.sup.4                                                                   3,6 × 10.sup.4                         ______________________________________                                    

A: Heat transfer coefficient without expanded graphite thermal transferlayer.

B: Heat transfer coefficient with thermal transfer layer according tothe invention.

EXAMPLE 2

Example 2 is identical to Example 1, with the sole difference that theexpanded graphite is replaced by another graphite (grade 5890 of LeCarbonne Lorraine having an expansion coefficient of 4.5 . 10 ⁻¹⁰ .K ⁻¹.Table 2 in a similar manner shows the results of the tests carried outin the same conditions as those in Example 1.

                  TABLE 2                                                         ______________________________________                                        kPa                                                                           pressure   10           80       120                                          ______________________________________                                        A' in                                                                         W.m.sup.-2.K.sup.-1                                                                      5 × 10.sup.3                                                                         6 × 10.sup.3                                                                     9 × 10.sup.3                           B' in                                                                         W.m.sup.-2.K.sup.-1                                                                      10.sup.4     2 × 10.sup.4                                                                     3 × 10.sup.4                           ______________________________________                                         A': Heat transfer coefficient without expanded graphite thermal transfer      layer                                                                         B': Heat transfer coefficient with thermal transfer layer according to th     invention.                                                               

A': Heat transfer coefficient without expanded graphite thermal transferlayer.

B': Heat transfer coefficient with thermal transfer layer according tothe invention.

In these tests, it is established that the heat transfer coefficientswith a contact according to the invention are on the order of or aregreater than 10⁴ W.m ⁻² .K⁻¹. With other pairings of materials and/ordifferent conditions, they reach values of 6×10⁴ W.m ⁻². K⁻¹.

FIGS. 1 to 5 illustrate application of the invention to the cooling of astructure subjected to intense, continuous, intermittent or pulsatingheat flux. To make the drawings clearer, the thickness of the walls ofthe metal tubes and layers of flexible material is greatly exaggerated.

In FIG. 1, the structure 1 to be cooled comprises a plurality ofpassages 2 into which are inserted metal tubes 3, which allow thecirculation of a cooling fluid (liquid or gas). The thermal transferlayer between structure 1 (which, for example, may be a block ofgraphite), and metal tube 3 is ensured by the use of a thin layer 4 offlexible material which is a good conductor, which may be expandedgraphite, more or less recompressed or rolled. For the installation,sufficient play is provided between metal tube 3 and passage 2, and thetube is surrounded with a layer 4 of expanded graphite. When theinstallation has been completed, the metal tube is subjected to anexpansion which may be obtained by placing it under hydraulic pressure,which causes the compression of expanded graphite layer 4 and reducesits thickness to a value which can be between 0.1 to 2 mm.

The pressure to which strip 4 is subjected must be equal at least to 10kPa, so as to ensure a heat transfer coefficient equal at least to 10 ⁴W·m ⁻² K ⁻¹.

In FIG. 2, a transverse cross section shows an element of structure 5which is graphite, in parallelepipedic shape, comprising two coolingtubes 3 of which the thermal contact with graphite block 5 is ensured byinterposition of a layer of expanded graphite 4, having a thicknessreduced by approximately 10% as a result of the expansion of tubes 2after their installation.

FIG. 3 shows a variation of embodiment wherein the thermal flux isapplied by an external fluid 6, which is contained by a first wall 7,which is preferably a metal wall in order to guarantee the seal; thethermal contact between this wall 7 and coolant tube 3 is ensured, asbefore, by interposition of a sheet 4 of expanded graphite, compressedfollowing its installation by expansion of tube 2, the sheet of expandedgraphite thus compressed having a final thickness on the order of 0.2mm.

FIGS. 4 and 5 show a variation of application of the invention whereinthe structure to be cooled is placed on the nest (bundle) of tubesfollowing interposition of the layer of flexible material.

In the example which is shown, the structure to be cooled is constitutedof an assembly by discrete elements such as bricks 8 of carbonaceousmaterial (graphite, or carbon-carbon composite), in which semi-circularpassages are preformed or worked in and the semi-circular passage areplaced on tubes 3 with interposition of flexible material 4. Thecompression of flexible material 4 is carried out by the means used forplacement and immobilization of bricks 8, means can be of any known type(stirrups, threaded rods, etc...).

Such a structure for instance can constitute the first wall of a fusionthermonuclear reactor for toric shape, which is directly subjected tothe heat flux generated by thermonuclear reactions.

The use of expanded compressed graphite to constitute the flexibleelement 4, because of its anisotropic structure, also presents theadvantage of ensuring spreading of the heat flux moving perpendicular tothe direction of transmission. In this manner, a local heat peak ("hotpoint") on the external surface of the structure to be cooled is spreadout over an extended peripheral zone of the cooling tube, on account ofthe high conductivity of the expanded compressed graphite in thedirection parallel to the thin sheets of graphite, in other words in thedirection perpendicular to the compression and the heat flux.

According to the invention, it is possible to use other forms offlexible carbonaceous materials having comparable thermal propertiessuch as woven materials or felts of carbon or graphite fibers,optionally charged with metal powder in order to improve the thermalconductivity. Considering their fibrous structure, these materials alsohave a highly anisotropic thermal conductivity.

The invention can be applied to any time intense, continuous,intermittent or pulsed heat fluxes that must be evacuated by fluidcirculation cooling means, and particularly in fusion or fission nuclearreactors (protective tiles, limiters and diverters), the continuouscasting of metals, and targets subjected to intense fluxes of rays orparticles which can be at levels, for example, of one to severalhundreds of watts per square centimeter.

What is claimed is:
 1. In a structure subjected to an intensecontinuous, intermittent or pulsating heat flux, a cooling meanscomprising fluids circulation tubes placed in passages in saidstructure, each of said tubes being surrounded by a layer of a heatconducting flexible carbonaceous material compressed between said tubeand said structure, the heat transfer coefficient between said tube andsaid structure being at least 10⁴ Wm⁻² K⁻¹.
 2. Structure as in claim 1,wherein said fluid circulation tubes are metallic.
 3. Structure as inclaim 1 or 2 wherein the flexible material 4 comprises expandedgraphite.
 4. Structure as in claim 3, wherein the expanded graphite ischarged with a metal powder.
 5. Structure as in claim 1 or 2 wherein theflexible carbonaceous material 4 is selected from the group consistingof woven materials and felts of carbon fibers and graphite.
 6. Structureas in claim 5, wherein said fibrous carbonaceous material is chargedwith metal powder.
 7. Process for fabricating a cooling device for astructure comprising a plurality of passages which is subjected to anintense continuous, intermittent or pulsating heat flux, comprising thesteps of:(a) surrounding each of a plurality of fluid circulation tubeswith a layer of heat conducting, flexible material; (b) inserting eachsaid surrounded tube into a passage in said structure; and (c)subsequently subjecting each said surrounded tube to an expansion underpressure sufficient to cause each layer of said flexible material to becompressed between said structure and the corresponding tube with aforce of at least 10 kPa.
 8. Process for fabricating a cooling devicefor a structure comprising a plurality of discrete elements which issubjected to intense continuous, intermittent or pulsating heat flux,comprising the steps of:(a) forming at least one semicircular passage ina surface of each of said elements; (b) lining each said passage with aflexible, heat conducting material; (c) nesting a cooling tube in eachsaid liquid passage; and (d) joining said elements and tubes so as toplace said heat conducting material under a compressive force of atleast 10 kPa.
 9. Process as in claim 7 or 8, wherein the flexiblematerial 4 is expanded graphite.
 10. Process as in claim 9, wherein theexpanded graphite is charged with a metal powder.
 11. Process as inclaim 7 or 8, wherein the flexible material 4 comprises a fibrouscarbonaceous material selected from the group consisting of wovenmaterials and felts of carbon fibers and graphite.
 12. Process as inclaim 11, wherein said fibrous carbonaceous material is charged withmetal powder.
 13. Process as in claim 8 or 8, wherein said structurebeing cooled is a wall of a thermonuclear fusion reactor.